1. Field of the Invention
This invention relates to an adaptive control system for an internal combustion engine and also to a method of controlling such an engine.
2. Discussion of Related Art
In operating an internal combustion engine, it is necessary to establish values for various control parameters and, depending upon the particular control parameter, the value of the parameter may be varied continuously in accordance with one or more operating parameters of the engine.
In a spark ignition engine, for each ignition spark or engine fire in one of the engine cylinders, an ignition timing parameter in the form of the spark advance angle must be established at the right moment to produce the peak combustion pressure soon after the piston has passed the top dead centre position so as to produce the optimum contribution to the output of the engine. Because the flame speed varies with the density of the air/fuel mixture, it is generally necessary to increase the spark advance angle with decreasing cylinder filling pressure. The spark advance must also be increased with increasing engine speed so as to allow for extra rotation of the engine crank shaft whilst the air/fuel mixture burns.
Until recently, the spark advance angle was established by a mechanical device responsive to manifold depression and engine speed. Such a mechanical device establishes the spark advance angle as a simple function of engine speed and load demand as represented by the manifold depression. Careful testing of engines shows that the ideal spark advance angle is a complex function of load and speed and this function cannot be matched by a mechanical device. Modern ignition systems now use empirically derived characteristics for the spark advance angle which are stored as a look up table in a read only memory.
These spark advance characteristics are determined by testing a number of samples of an engine and establishing an optimum spark advance angle for each load/speed point. A value for the spark advance angle for each point is then chosen which will give good economy subject to meeting various constraints such as low emissions and low knock levels.
Although this provides a much closer match to the optimum spark advance angle than was achieved with the mechanical devices, it still does not give the engine user the best possible spark advance angle for his engine throughout its life. There are a number of reasons for this. It is not possible to test enough engines to provide good statistics and the engines available during tests are often different from production engines. Also, variations in the engine characteristics may occur due to manufacturing tolerances and from small changes in engine design. During the life of an engine, various ageing effects will occur in the engine and in the sensors, actuators and electronic circuitry and these will create a mismatch between the optimum characteristics and those stored in the read only memory.
In U.S. Pat. No. 4,379,333, there is described an adaptive control system for controlling the spark advance angle. In this system, small positive and negative perturbations are superimposed on the spark advance angle and the resulting changes in engine speed are used to determine the differential or slope of engine output with respect to spark advance angle.
According to the sign of the slope measurement, a correction is then made to a schedule of spark advance angles. The slope measurement is then repeated and the first slope measurement is replaced by a completely new measurement without any reference to the first measurement of the slope. The new measurement is used to make a further correction to the schedule of spark advance angles.
If the load and speed have remained steady, the same element in the schedule will be additively corrected in correspondence to successive slope measurements. Thus, information gathered in previous measurements of the slope is retained in the schedule element for a while, this information being progressively diluted as new data becomes available.
If the slope of engine output with respect to spark advance is positive, the values for the spark advance angle will be progressively increased until eventually the maximum value of engine output is achieved or exceeded and the slope measurements decay to zero or change sign. The correction process will then be driven only by errors in the slope measurements which will cause small random wanderings in the values of the spark advance angle about the optimum value.
The general form of the function relating engine output to the spark advance angle is known to be a fairly simple curve with a single maximum of engine output. Because the system of U.S. Pat. No. 4,379,333 drives the spark advance angle towards the value which gives maximum engine output, some knowledge of this relationship is implicit in the design of this system. However, because the system of U.S. Pat. No. 4,379,333 corrects the spark advance angle in a progressive manner rather than making an estimate of the spark advance angle which gives maximum engine output, the system does not take full advantage of knowledge of this relationship.
Under certain engine operating conditions, such as idling, it is sometimes necessary to reduce the spark advance angle from the value which gives maximum engine output. This is done in order to reduce emission of pollutants or to reduce knock. Under such conditions U.S. Pat. No. 4,379,333 inhibits correction of the spark advance angle. Thus, under such operating conditions, U.S. Pat. No. 4,379,333 takes no advantage of knowledge of the relationship between spark advance angle and engine output and loses the advantage inherent in an adaptive control system.
Similar considerations to those discussed above apply to other engine control parameters, such as mixture composition in the case of a spark ignition engine, or injection timing or amount of fuel injected in the case of a compression ignition engine.